Heat treating furnace



Dec. 15, 1936. E. v. GILBERT HEAT TREATING FURNACE 2 Sheet-Sheet FiledDec. 24, 1931 [l ZHVEHUR Dec. 15, 1936. E. v. GILBERT HEAT TREATINGFURNACE Filed Dec. 24, 1951 2 Sheets-Sheet 2 i: z: ETJ/EJA;

Patenta!`-` Dec. 15, 1936 UNITED STATES PATENT OFFICE HEAT TREATINGFURNACE Erwin V. Gilbert, Hartini-d, Conn.: Alice H. Gilbertadministratrix o! said E'win V. Gilbert,

deceased The general object of the invention is to eliminate or reduceto a minimum oxidation or other deleterious chemical effects upon thesteel or other material being treated. This result is attained not onlyby controlling the chemical con- V tent of the atmosphere within thework containing or heating chamber of the furnace, but also bymaintaining the said atmosphere in a substantially quiescent condition.The invention is apo plicable to various types of heat treatingfurnaces, but for reasons which will presently appear it is speciallyapplicable to a fuel heated furnace.

In the heat treatment of metals it has long been recognized thatatmospheric air should be excluded from the heating chamber; and it hasheretofore been common practice to provide a furnace having a heatingchamber which is open at the front for the introduction and renoval ofthe work and which is 'otherwise closed so as to .exclude thesurrounding gases, and to provide a curtain of upward moving hot gasesacross the passageway at the front of the chamber to thus prevent, atleast in part, the entry of atmospheric air into the chamber.

Even when .the atmbsphere in the heating chamber is substantially freefrom uncombined oxygen, chemical reactions may take place between theambient gases and the steel or other material being heated. This fact iswell exemplified by the common practice of introducing into the heatingchamber sufi'cient proportion of carburizing gases to preventdecarburization, and by the practice of introducing neutral gases forthe purpose of preventing or at least minimizing chemical reactionbetween the chamber atmosphere and the material being heated. Onlylimited success has attended these attempts to adjust or control theatmospheric content of the heating chamber in such a way as to preventchemical action. This lack of success has been due in part to thevariations in chemical activity resulting' from changes in temperature,and has also been due in part to the difiiculty of accurately con- 45trolling the composition of the chamber atmosphere,-particularly whenthe said atmosphere in accord with common practice comprises burninggases and the end products of combustion.

While it has been recognized that air or other 50 gases containing freeoxygenshould be excluded from the heating chamber, and while it has beenrecognized that the atmospheric content of the chamber may be otherwisecontrolled to minimize chemical activity, little or no consideration 55has been given to the fact that the atmosphere in the chamber regardlessof its composition should be maintained in a substantially tranquil orquiescent state. Observation of steels at elevated temperatures hasdisclosed the fact that little or no chemical activity takes place whenthe ambient atmospher is substantially quiescent, even though the saidatmosphere may contain chemically active constituents such as oxygen,hydrogen, sulphur compounds and the like. This is believed to be due toa protective gaseous envelope which surrounds the work and which iseither exuded from the steel or is formed by chemical reaction betweenthesteel and the gaseous atmosphere. This *protective envelope iseffective only when the ambient atmosphere is quiescent and it isdisturbed and made ineffective by even a slight degree of motion orturbulence.

Heretoore, in heat treating furnaces utilizing atmospheric control, theneutral gases or the more or less controlled gases of combustion havebeen freely and continuously introduced into the heating chamber inlarge quantities, thus causing and i maintaining a high degree ofturbulence with resulting extensive chemical activity, which activitywas particularly great when there was any considerable departure from acondition of chemical eguilibrium. This sustained turbulence hasprevented the functioning of the protective gaseous envelope heretoforedescribed, and has made necessary an accuracy of atmospheric controlwhich has been impossible of attainment.

In accordance with the present invention I very completely excludeatmospheric air from the heating chamber of the iurnace, and Iaccomplish this result in such a way that the atmosphere within thefurnace is disturbed to a minimum extent and is allowed to remain in asubstantially tranquil quiescent condition, thus permitting theprotection of the metal by the inherent auto-protective action alreadydescribed.

For the purpose of controlling the atmospheric 'content of the workingchamber it has been heretofore proposed, particularly in conjunctionwith electrically heated furnaces, to provide a fuel burner of one typeor another at or near the front opening to the working chamber so as todeliver a curtain of moving burning or burned gases, which curtainextends across the opening and, to a grater or less extent, prevents theentry of atmospheric air into the chamber, but introduces other gases.

In accordance with prior practice a large portion of the gasesconstituting the protective curtain has been guided directly into theheating chamber, thus causing a continuous and sustained turbulencetherein; but in accordance with the present invention the gases of thegaseous curtain are so guided and controlled that they are preventedfrom entering the heating chamber to any considerable extent, thusleaving the atmosphere in the chamber substantially quiescent, with theresultant advantages already pointed out.

It is generally 'recognized that electrically heated furnaces havecertain important advantages, but that such furnaces are neverthelesssubject to the rather serious disadvantage that the initial cost and thecost of operation and maintenance are considerably greater than tor fuelheated furnaces of equal capacity. The present invention may beadvantageously incorporated in a fumace of the less expensive and moreeconomical fuel heated type, and in accordance with one phase of theinvention the products of combustion from the fuel are directed, guidedand controlled so as to serve in and of themselves as the means forpreventing the entry of deleterious gases into the working chamber, andso as to otherwise serve to reliably and emciently control theatmospherlc condition within the said chamber. In accordance with myinvention I not only attain the advantages that are inherent in afuelheated furnace, but I am enabled to also provide without additionalexpense of operation a means of atmospheric control which is much moreeffective and reliable than is any means heretofore provided tor thispurpose.

A further object of the invention is to provide in combination with themeans for excluding atmospheric air and other deleterious gases, a meansfor introducing into the muiiie suitable selected gases adapted tochemically combine with or effect a neutral reaction upon the work beingheated and treated.

Still another object of the invention is to provide a furnace of thetype referred to having openings at opposite ends of the muffie orworking chamber so that the work to be treated can be introduced at oneend andremoved at the other end. This Construction makes it possible fora conveyor to be extended through the muflie so that the work can bemoved continuously therethrough.

'Other objects of the invention will be apparent from the followingspecification and claims.

In the accompanying drawings I have shown alternative constructionsadapted for attaining the foregoing and other objects. The drawings showfurnaces of the fuel heated type, but as to its broader phases theinvention is not so limited. It will be understood that the drawings areintended for illustrative purposes only and are not to be construed asdeflning or limiting the scope of the invention, the claims forming apart of this specification being relied upon for v that pu pose.

Of the drawings:

Fig. 1 is a plan view of a furnace embodying the invention.

Fig. 2 is a. vertical longitudinal sectional View taken along the line2-2 of Fig. 1.

Fig. 3 is a vertical transverse sectional view taken along the line 3-3of Fig. 2.

Fig. 4 is a horizontal sectional view taken along the line 4--4 of Fig.2.

Fig. 5 is a combined plan and sectional 'view showing an altemateembodiment of the invention, the section being taken along the line 5-5of Fig. 6.

Fig. 6 is a vertical longitudinal sectional view taken along the line6-6 of Fig. 5.

Fig. 7 is a combined plan and sectional view showing a second altemateembodiment of the invention, the section being taken along the line 1- 1of Fig. 8.

Fig. 8 is a vertical longitudinal sectional View taken along the line8-8 of Fig. 7.

Fig. 9 is a vertical transverse sectional view taken along the line 9-9of Fig. 8.

Fig. 10 is a plan View of a furnace embodying the invention and havingopenings therein at opposite ends.

Fig. 11 is a vertical longitudinal sectional view takenalong the lineH-Il of Fig. 10.

Referring first to Figs. 1 to 4 of the drawings, I represents as anentirety the external casing of the furnace, which casing is lined withfire brick or other heat resisting and insulating material. The frontwall of the furnace is provided with an opening or passageway 2 throughwhich work to be heated is put in place and removed. Mounted within thefumace is a muflle or' heating chamber 3, the front end of which is openand is in register with the passageway 2 in the front wall. It will beobserved that, except for the front, the heating chamber 3 issubstantially completely closed so as to exclude surrounding gases fromthe interior thereof. Preferably cheek pieces 4, 4 are provided at thesides of the passageway 2, these cheek pieces constituting inwardextensions of the passageway and engaging the side walls of the mufi'ie.The muflle is supported on pedestals 5, 5 and 6 which project upwardfrom the floor of the furnace.

In accordance with the invention there is provided a source of hotgases, which gases are r ordinarily gases of combustion. Communicatingwith the said source of hot gases is a duet which terminates -in anarrow transverse slot in one wall of the passageway 'and thus causes acurtain of the said hot gases to continuously move across the passageway2 at the front of the heating chamber. Ordinarily the slot extendsacross the bottom-of the passageway as indicated at 'I and the gasesmove upward; but as to this I. do not limit myself. As shown the muflleor heating chamber 3 is mounted in spaced relation with the front wallof the furnace, the said slot 'l being formed between the muille and thesaid wall. The hot gases directed through the slot 1 move upward asaforesaid and thus form a gaseous curtain at the front of the heatingchamber. This curtain, particularly when it consists of gases ofcombustion, serves to prevent oxygen or other gases from the externalair from entering the heating chamber.

In order to denitely regulate and control the gases delivered throughthe slot- 1 I provide means associated with the wall of the passagewayopposite the said slot 'l for substantially preventing the movement ofthe curtain gases into the heating chamber. Preferably and as shown thesaid means comprises a recess 8 in thewall of the passageway oppositethe slot 1, and when the said slot is at the bottom as shown the recess8 is at the top of the passageway. This recess 8 is so positioned and soformed as to be adapted for receiving the upward moving hot gases, andit prevents the said gases, or any substantial portion of them, frombeing deflected or diverted inward into the heating chamber. Preferablythe width of the recess 8 is considerpermitting the gases to freelyenter notwithstanding expansion that may have taken place as the gases-travelled across the passageway. It will be observed that the innerwall 8' of the recess &constitutes a transverse bame facing outward awayfrom the chamber and that this bafile serves to engage the gases andprevent their movement horizontally into the chamber. In this way theinherent turbulence of the gases constituting the curtain issubstantially confined to the outside of the heating chamber, thusassuring approximate tranquility and quiescence within the said chamber.

Preferably in addition to the recess 8 I provide an exhaust outlet 9 forcarrying the hot gases away from the passageway 2. As shown in Figs. 1to 4 the exhaust o-utlet 9 communicates directly with the recess 8 andextends upward the'efrom. The exhaust outlet may, however, be otherwiselocated as will be presently explained.

When the exhaust outlet 9 communicates directly with the recess 8 asshown in Figs. 1 to 4 the initial duct terminating in the said slot 1,the said recess 8 and the said exhaust outlet 9, may be consideredcollectively as constituting an interrupted fiue having portions thereofoppositely positioned with respect to the passageway 2 and serving toconduct hot gases across the passageway in front of the heating chamber.As already stated the recess 8 is of considerably greater width than theslot "I, thus permitting the gases of the curtain to expand. This freeexpansion of the gases relieves any pressure which might otherwise beformed in front of the heating chamber. Preferably and as shown thewalls of the recess 8 converge, thus giving i-ncreased velocity to theupward moving gases, and the walls of the outlet 9 immediately above therecess diverge, thus giving decreased velocity tothe said gases. highvelocity and a reduced pressure at the zone between the recess 8 and theoutlet 9;

The gaseous curtain not only serves to prevent the admission ofdeleterious external gases, but it probably also serves to form a slightvacuum in the heating chamber.

Preferably and as already stated the furnace is heated by the directcombustion of fuel therein. To this end the rear wall of the furnace isprovided with an opening o at which is located a burner Il through whichliquid or gaseous fuel may be projected into the in'terior of thefurnace. Preferably there is 'provided at the top of the furnace anopening IZ which may be used for ignition purposes when a gaseous fuelis used. If liquid fuel is used it may be ignited directly at the burnerll. Ordinarily the opening |2 is closed by a removable cap !3. It willbe observed 'that the main walls of the furnace constitute a combustionchamber which is separate from the heating chamber but which is sopositioned that the heat of combustion is transmtted to the heatingchamber. As shown the combustion chamber substantially surrounds theheating chamber except at the front therecf.

The main combustion chamber of the furnace preferably constitutes thesource of the hot gases which are delivered in such a way as to form agaseous curtain at the front as already described. Preferably the gasesare delivered a t the bottom through a slot such as that at 1. To thisend the combustion chamber is formed with a portion which extendsforwardunder the heating chamber and which may be regarded as a ductterminating in the before-mentioned slot 1. A wall |4 The gases are thusgiven a relatively separates the recess 8 and the outlet 9 from thecombustion chamber, this wall extending downward at the sides asindicated at M', |4'. I have already pointed out that the gas passagesat the front may be regarded as collectively constituting an interruptedfiue, and thus takes the place of the conventional exhaust fiue. It willbe apparent that this ue at the front communicates with the combustionchamber and serves toconduct the hot exhaust gases from the combustionchamber across the passageway 2 in front of the heating chamber 3, thusforming a curtain of moving hot gases.

When the hot gases of combustion are supplied through the slot 'I at arate less than normal they may fail to sufiiciently protect the interiorof the heating chamber from the entry of external gases. This may happenwhen the fuel burner is adjusted to provide only a very low heat. Underthese circumstances it is preferable to provide a supplemental .supplyof gas at the side of the passageway 2 opposite the slot 1, that is, inthe present instance, at the top of the passageway. The gases sosupplied at opposite sides of the passageway will cooperate with eachother to provide a body of moving protective gases at the front of theheating chamber. As a result of the additional protection provided bythe gases supplied at the top it is possible to reduce the temperaturatwithin the heating chamber without any infiltration of external gases.

When the gases are drawn from the main combustion chamber as ispreferred I suppiy the said gases to the top of the passageway 2 bymeans of secondary ducts !5, stantially horizontally through the wall !4and which terminate at the front in the recess 8. When the furnace isOperating in a normal manner the secondary ducts 15, !5 are lessimportant.

Ordinarily it is unnecessary, and in fact impracticable, to cause all ofthe gases of combustion to flow through the narrow slot l. Some of theremaining gases may pass through the secondary ducts |5. 15; but Ipreferably also provide auxiliary ducts Hi, IB which are locatedadjacent the respective sides of the passageway 2.4 These auxiliaryducts !6, IE communicate with the lower portion of the combustionchamber by means of slots l 1, I 'I and they extend to and merge withthe recess 8. The gases are guided into the slots Il, I'I by triangulardeflectors I 8, !8.

At !9 I have shown a tube extending through the rear wall of the furnaceand also through the rear wall of the mutfle or heating chamber 3. Thistube may be used for the introduction of a thermocouple'of any usual orpreferred construction.

In addition to the tube |9 referred to I may also provide a pipe 20which also extends through the rear wall of the furnace and through therear wall of the muffle or heating chamber 3.

The pipe 2& is provided with a valve ZI and the pipe may be connectedwith any suitable source of gas. By means of the pipe 20 any selectedgas of predetermincd character may be admitted to the interior of theheating chamber, the rate of admission being controlled by the valve 2!or other suitable controlling device. The gas so *admitted Will contactthe material in the furnace, the type of reaction being depenclent uponthe gas used, upon the material being treated and upon the temperatureinvolved. The introduction of a selected gas in the manner describedbecomes practicable by reason of the fact that the entry of deleteriousgases at the front |5 which extend submay be substantially prevented byproperly regulating the gaseous curtain in the manner already described.The maintenance of a slight vacuum within the heating chamberfacilitates the flow of the selected gas thereinto and also causes thegradual withdrawal of the selected gas at the front, therebyinsuring anundiluted amosphere of the said gas.

With a gaseous protective curtain formed as herein described it isordinarily unnecessary to provide a door for closing the front end ofthe passageway to the heating chamber. If desired however a door may beprovided, and in Figs. and 6 I have shown an alternative constructionwhich includes a door. The Construction of the furnace is, in the main,similar to that shown in Figs. 1 to 4, and a complete detaileddescription is unnecessary. At thefront of the furnace there is provideda door 22 which is vertically movable in guideways 23, 23. In itslowermost position the door rests upon a shelf 24. In the construetionas shown in Figs. 5 and 6 there is provided a recess 8 which is similarto the recess 8 but which is entirely closed at the top except forauxiliary ducts |5 leading from the main combustion chamber. The exhaustoutlet 9 instead of extending directly from the top of the recess 8 islocated in front of the said recess communicating directly with thepassageway 2 at the front of the recess. Preferably and as shown theoutlet s is formed as a depression in the rear face of the door 22, thepassageway thus being positioned between the said door and the mainfront wall of the furnace.

Gases are guided` to the slot 'l by means of a beveled deflector 25.Gases are also permitted to rise at the sides of the front passagewaythrough auxiliary ducts !6 Is Slots H l'l are provided to facilitate theentry of gases 'into these auxiliary ducts.

The operation of the furnace shown in Figs. 5 and 6 is substantially thesame as already described, with the single exception that the gasesinstead of fiowing directly upward from the recess & circulate throughthe said recess and then forward along the top wall of the passageway 2to the exhaust outlet 9 v In Figs. 7 to 9 I have shown a Constructionwhich is similar to that shown in Figs. 1 to 4 and which diiferstherefrom primarily in the arrangement of the fuel burners. ThisConstruction is intended primarily for the use of gaseous fuel andinstead of a single bumer I I at the rear there are provided multipleburners located at the sides. As shown there are two burners 26, 26located at the left side of the furnace near the bottom. Similar burners21, 21 are located at the right side near the top. Gas is supplied tothese burners 'through piping indicated at 28 and 29. The gases ofcombustion from the said burners 25 and 21 tend to circulatecircumferentially around the muflie a and in order to permit such freecirculation the muffie is supported upon narrow slabs 5 5 and 6. Thefiue construction at the front for the passage of gases is similar tothat already described and repetition of the description is unnecessary.It will be observed that the auxiliary ducts IS, Is at the sides areconsiderably larger than the ducts IS, IS before described.

In Figs. 10 and 11 I have shown a construction similar to that shown inFigs. 1 to 4, with the exception that the muflie3 is open at both ends,similar passageways 2 and 2 being provided in communication with thesaid open ends. The fiue constructions at both ends of the furnace maybe the same as already described, and a repetition of the detaileddescription is therefore unnecessary.

With a furnace Construction as shown in Figs. 10 and 11 it is possiblefor the articles to be treated to be moved longitudinally through themuifie, being inserted through one passageway, as for instance thepassageway 2, and being discharged through the opposite passageway, asfor instance the passageway 2. The gaseous curtains such as alreadydescribed prevent the admission of deleterious gases at either end ofthe mufiie.

When the furnace is constructed with two open ends as shown it may beprovided with a conveyor having an endless link belt such as 30extending through the muflle. This belt can be connected with anysuitable source of power so as to be moved at the desired rate in thedirection indicated. The articles to be treated can be placed on thebelt at the entrance end and will then be carried through the heatingchamber and finally removed or discharged at the opposite end.

What I claim is:

1. In a heat treating furnace having a heating chamber and a passagewayat the front communicating with the heating chamber, the combination ofa duet adapted for supplying hot gases and terminating in a narrowtransverse slot extend-. ing across the bottom of the passageway so asto form a curtain of upward moving gases at the front of the heatingchamber, and means additional to the said duet and slot for supplyinghot gases at the top of the passage above the slot.

2. In a heat treating furnace having a heating chamber and a passagewayat the front communicating with the heating chamber, the combination ofa duet adapted for supplying hot gases and terminating in a narrowtransverse slot extending across the bottom of the passageway so as toform a curtain of upward moving gases at the front of the heatingchamber,- a recess provided at the top of the passageway above the duetadapted for receiving the said upward moving hot gases and having awidth greater than the width of the slot, and means additional to thesaid duct and slot for supplying hot gases to the said recess.

3. In a heat treating furnace having a. heating chamber and apassageway' at the front communicating with the heating chamber, thecombination of a duet adapted for supplying hot gases of combustion andterminating in a narrow transverse slot extending across the bottom ofthe v passageway so as to form a curtain of upward moving gases at thefront of the heating Chamber,` a recess being provided at the top of thepassageway above the duet adapted for receiving the said upward movinggases, and an upward extending exhaust outlet separate from the recessand communicating with the passageway at the front of the said recess. 4

4. A heat treating furnace as set forth in claim 3, wherein there is amovable door at the front end of the passageway, an'd wherein the saidexhaust outlet is formed between the door and the front wall of thefurnace.

` 5. A heat treating furnace comprising in combination, a heatingchamber, a passageway at the front communicating with the heatingchamber, and an interrupted flue for conducting hot gases of combustion,which fiue includes a supply duct terminating in a narrow transverseslot extending across th'e bottom of the passageway and also includes arecess at the top of the passageway which is of considerably greaterwidth than the said siot so as to permit the said gases to expandtherein. i

6. A furnace as set forth in claim 5, wheren the walls of the recessconverge thus giving increased velocity to the upward moving gases, andwherein the walls of the flue above the recess diverge thus givingdecreased velocity to the said gases.

7. A furnace as set forth in claim 5, wherein there is a second duct forsupplying additional hot gases of combustion to the said recess.

8. A heat treating furnace comprising in combination, a heating chamber,a passageway at the front communicating with the heating chamber, acombustion chamber separate from the heating chamber but positioned sothat the heat of combustion is transmitte'd to the said heating chamber,a duct communicating With the combustion chamber and terminating in anarrow transverse slot extending entirely across the said passageway,the said duct serving to conduct the hot gases of combustion from thecombustion chamber to the passageway so as to form a moving gaseouscurtain at the front of the heating chamber, and means for supplyingadditional hot gases from the combustion chamber to the passage at aposition opposite the slot.

9. A heat treating furnace comprising in combination, a heating chamber,a passageway at the front communicating with the heating chamber, acombustion chamber substantially surrounding the heating chamber exceptat the front, a duct communicating With the combustion chamber andterminating in a narrow transverse slot extending entirely across thebottom of the said duct serving to conduct the hot gases of combustionto the passageway so as to form an upward moving gaseous curtain at thefront of the heating chamber, and auxiliary ducts communicating with thelower portion of the combustion chamber and extending upward adjacentthe respective sides of the passageway.

10. The combination in a heat treating furnace, of a heating chamber, apassageway at the front communicating with the heating chamber, acombustion chamber separate from the heating chamber but positioned sothat the heat of combustion is transmitted to the said heating chamber,a duet communicating with the combustion chamber and terminating in anarrow transverse slot extending across the bottom of the passageway soas to form a curtain of upward moving gases at the front of the heatingchamber, a recess being formed at the top of the passageway above theduct for receiving the said upward moving gases, and a* second ductcommunicating with the combustion chamber and terminating directly inthe recess.

11. The combination in a heat treating furnace, of a heating chamber, apassageway at the front communicating with the'heating. chamber, acombustion chamber substantially surrounding the heating chamber exceptat the front, and an interrupted flue communicating with the combustionchamber and including a'supply duct terminating in a narrow transverseslot extending across the bottom of the passageway, the said flue alsoincluding a recess at the top-of the passageway which is of considerablygreater width than the said slot thereby permitting the said gases toexpand therein.

12. A furnace as set forth in claim 11, wherein there is a second ductcommunicating with the combustion chamber and. terminating directly inthe recess. v

13. A furnace as set forth in claim 11, wherein there are auxiliaryducts communicating with the lower portion of the combustion chamber,the said auxiliary ducts extending upward adjacent the respective sidesof the passageway and terminating in the said recess.

14. In a heat treating furnace having a heating chamber and a passagewayat the front communicating with the heating chamber, the combination ofan interrupted flue having separated portions positioned with thepassageway between them so as to serve to conduct a curtain of hot gasesof combustion across the passageway in front of the heating chamber,which curtain excludes deleterious gases from the external atmosphere,and means for supplying directly to the interier of the heating chambera gas of predetermined character adapted to contact the articles beingtreated.

15. In a heat treating furnace having a heating chamber and a passagewayat the front communicating with the heating chamber, the combination ofa combustion chamber separate from the heating chamber but positioned sothat the heat of combustion is transmitted to the said heating chamber,an interrupted flue communicating with the combustion chamber and havingportions below and above the passageway, the said flue serving toconduct a curtain of hot exhaust gases from the combustion chamberacross the passageway in front of the heating chamber, which curtainexcludes deleterious gases from the .external atmosphere, and means forsupplying directly to the interior of the heating chamber a gas ofpredetermined character adapted to contact the articles being treated.

16. A heat treating furnace comprsing in conbination, a heating chamber,two similar passageways communicating with the heating chamber atopposite ends thereof, two ducts adapted for supplying hot gases ofcombustion and terminating in narrow transverse slots extending acrossthe respective passageways so as to form curtains of moving gases, tworecesses being formed in the walls of the respective passagewaysopposite the ducts for receiving the said upward moving gases, and twoexhaust outlets adapted to receive the gases discharged from therespective recesses.

17. A heat treating furnace comprising in combination, a heatingchamber, two passageways communicating with the heating chamber atopposite ends thereof, a combustion chamber separate from the heatingchamber but positioned so that the heat of combustion is transmitted tothe said heating chamber, and two ducts communicating with thecombustion chamber and terminating in narrow transverse slots.ext'ending entirely across the respective pas'sageways, the said ductsserving to conduct the hot gases of combustion from the combustionchambers to the passageways so as to form moving gaseous curtainspositioned across the passageways. v

18. A heat treating furnace comprising in combination, a heatingchamber, two passageways communicating with the heating chamber atopposite ends thereof, a combustion chamber separate from the heatingchamber but positioned so that the heat of combustion is transmitted tothe said heating chamber, and two interrupted fiues communicating withthe combustion chamber and having portions below and above therespective passageways, each flue serving to conduct a curtain of hotexhaust gases from the combustion chamber across the correspondingpassageway.

chamber which is otherwise substantially completely closed to excludesurrounding gases from the interior thereof, the combination of a.transverse bame associated with one wall of the passageway and having asurface facing outward away from the chamber, and means extending acrossthe opposite wall of the passageway for projecting a curtain of movinghot combustion gases transversely of the passageway and along the saidoutward facing surface of the bame.

20. In a heat treating furnace, the combination of a heating chamberwhich is open at the front and which is otherwise substantiallycompletely closed to exclude surrounding gases from the interierthereof, and walls forming a passageway communicating with the openfront of the heating chamber, the bottom of the passageway having a slottherein extending from side to side for projecting upward across thepassageway a transverse curtain of moving hot combustion gases tothereby' exclude atmospheric air from the chamber and the top of thepassageway having a recess therein extending from side to side forreceiving the said gases and substantially preventing movement thereofinto the chamber.

21. In a heat treating furnace, the combination of a heating chamberwhich is open at the front and which is otherwise substantiallycompletely closed to exclude surrounding gases from the interiorthereof, and walls forming a passageway communicating with the openfront of the heating chamber, the bottom of the passageway having a slottherein extending from side to side for projecting upward across thepassageway a transverse curtain of moving hot combustion gases tothereby exclude atmospheric air from the chamber and the top of thepassageway having a gas-receiving recess therein opposite 'the slot andextending from side to side and havirlg a width considerably greaterthan that of the s ot.

22. In a heat treating furnace, the combination of a muffie which isopen at the front and which is otherwise substantially completely closedto exclude surrounding gases from the work containing chamber thereof,the fr'ont edges'of the mufiie being inclined upwarri and rearward, andwalls forming a passageway communicatin'g With the open front of themuflie, a transverse slotbeing formed between the bottom walls of thepassageway and mufile for projecting upward across the passageway atransverse curtain of moving hot combustion gases to thereby excludeatmospheric air from the chamber and there being a transverse recess inthe top of the passageway immediately adjacent the top wall of themuffle and wider than the said slot which recess is adapted forreceiving the said gases and substantially preventing movement thereofinto the chamber.

ERWIN V. GILBERT.

